Download - 6. R & D(1) mky
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Basic Overview of Powder Coatings
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Contents
History
Powder properties
Chemistries
Manufacture
Application processes
Color
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History
Early 1950s -first thermoplastic powders applied onto heated metal
Late 1950s -first thermosetting powders for pipeline protection (Shell)
1962 - dispersion via extrusion (Shell)
1964 - first epoxy-based decorative powder coatings
1964 - first dedicated powder spray equipment (Sames)
1970 - first polyester-based durable coatings. (Scado & UCB)
1970s oil crisis.
Growth rates from the 1970 - 1990s averaged 15% with peak growth rates often achieving 25%
2000 Growth in mature markets
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Features of powder
Zero VOC
Comply with environmental legislation
H&S, reduced fire risk
Densely crosslinked coating, high Tg
Good barrier & adhesion properties
Easier equipment cleaning and removal of uncured overspray
Over-sprayed powder can be recycled
high usage rates (95%)
reduced air pollution & effluent
Usually cured by thermal energy
conventional powders unsuitable for heat-sensitive substrates like wood, plastics
Film thickness dictated by particle size
generally thicker than wet paint films (50-80m)
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Coating Properties
100% Solids no VOCs
Thermosets 150-200C for 5-20 mins
Film Thickness 50-100m
Melt Viscosity 10000 mPas
Product Tg 45-55C
Film Tg 60-110C
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Chemistry
99% thermosetting powders
Chemistry used in a product depends upon end use
requirements.
Main binder systems currently in use:
Polyester
UV resistance
Mechanical & chemical resistance only fair
Epoxy
Chemical resistance
Poor UV durability
Polyester/epoxy (hybrid)
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Powder chemistries
Main cross-linking reactions used in powder coatings are as follows:
Epoxy - carboxylic acid
Too slow for ambient-cure coatings but very widely used in powders
PE/TGIC, epoxy-polyester hybrids and many acrylics all use this scheme
N.B. TGIC is a mutagen; TOXIC labelling now required.
Carboxylic acid - hydroxyl
e.g. PE/Primid
Hydroxyl - isocyanate
Polyurethane powders comprise OH-functional polyesters crosslinked with isocyanate adducts
Epoxy - amine
Epoxy - phenolic
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Summary of Chemistries
Epoxy Epoxy
Polyester
Standard
Polyester
Superdurable PU Fluoro GMA
Acrylic
Acrylic
Hybrid
Exterior
Durability
Very
Poor
Poor Good Very Good Good Excellent Very
Good
Medium
Corrosion
Resistance
Very
Good
Very
Good
Good Good Good Good Good Good
Impact Very
Good
Very
Good
Good Poor Good Good Poor Poor
Flexibility Very
Good
Very
Good
Good Medium Good Good Good Good
Adhesion Very
Good
Very
Good
Good Good Very
Good
Medium Very
Good
Very
Good
Chemical
Resistance
Very
Good
Very
Good
Good Good Good Very
Good
Very
Good
Very
Good
Heat
Resistance
Very
Poor
Good Good Good Good Good Good Good
Abrasion Very
Good
Good Good Good Good Good Good Good
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Manufacture
Pre-weighing of dry RMs
Extrusion (melt mixing & homogenising)
Micronising (milling)
Classification (particle size control)
Collect finished powder
Pre-mixing
Off-line tint
QC colour & gloss mechanical properties particle size
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Powder Manufacture
Premix
Extrusion
Cooling
Kibbling
Milling
Sieving
Packing
Interpon
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Application Fluidized Bed Methods
Hot dipping
Earliest means of application (1950s). Immerse pre-heated substrate in
fluidized bed of powder
good all-over coverage
thick film build, poor control
Electrostatic fluidized bed
Bed contains electrodes which ionize fluidizing air; this in turn charges
powder
No pre-heating; 30-100kV
good coverage
poor control of film build
Faraday cage effects
electrical safety problems
Fluidized bed application is now little used
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Application Spray application
Most widely used means of application
Two types:
electrostatic (corona)
tribostatic (friction)
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Spray application Corona spray Powder gun contains electrode at c.50-100kV
Ionized air molecules charge powder particles, which deposit on
earthed workpiece
Easy control of film build
Back-ionization - repulsion of free ions trapped in deposited powder layer
Faraday cage effects - powder particles cannot penetrate complex shapes
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f i e l d l i n e s
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Spray application (contd.) Tribostatic (Tribo) Spray
Powder particles are charged by friction between each other and between
powder & gun
Magnitude of charge transfer depends upon:
materials used (triboelectric series)
intimacy of contact and residence time
particle size, shape and relative surface area
Tribo guns commonly have a PTFE lining (bottom of tribo series, all polymeric
materials charge +ve relative to PTFE)
No Faraday cage - only weak electric field
Back-ionization onset also delayed
High maintenance costs - gun wear
Lower powder throughput than corona
Difficulty charging fine powder
No Field Lines
+V
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Formation of cured film
Powder particles are retained on the substrate by electrostatic attraction
until the coating is cured
Various types of curing oven can be used:
convection oven (most common)
medium-wave infra-red
induction
Film formation process:
Powder
particles Particles melt &
coalesce
Crosslinking &
hardening
DH DH
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Typical Tests Gloss
Gloss measurements are made to
check that the gloss is within
allowable tolerances.
Typical gloss levels are:
Gloss over 80 %
Semi-gloss 66% - 79%
Satin 55% - 65%
Matt 20% - 30%
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Typical Tests Film Thickness
The powder coating is formulated to
be used at a specified target film
thickness..
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Typical Tests Adhesion by cross cut
A lattice pattern is cut through the coating
with a blade and tape iapplied over the
lattice and then removed rapidly.
Performance is measured by counting the
number of squares removed. Normal
standard is Gt0 (no removal on tape test)
if coating has been correctly applied and
cured fully
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Typical Tests Pencil Hardness
Is a measure of the hardness of the
coating.
Performance is measured as the hardest
pencil to give no rupture to the film
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Typical Tests Color Control
A spectrophotometer is used to assess
color.
Reflectance values are measured at
several wavelengths, computed and
calculations made in the computer.
Color differences are quoted in terms of:
Lightness DL
Red/Green Da
Yellow/Blue Db
Chroma (color strength) Dc
Hue Angle (shade) Dh
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Lightness
Description of color
Lightness
Light colors
Dark colors
Chroma
Strong colors
Weak colors
Hue
Red, green, yellow, blue
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Numerical Description of Color
CIEL*a*b* (Commission Internationale de lEclairage)
A Uniform Color Space
3 Dimensional
L* lightness/darkness
a* redness/greenness
b* yellowness/blueness
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Color Difference from standard
Total difference form a standard
DE*>1 the color difference is perceptible.
Lightness difference:
DL* is positive, the trial is lighter than the standard.
DL* is negative, the trial is darker than the standard.
DE = {(DL*2) + (Da*2) + (Db*2)}1/2
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Color Difference from standard
Difference on red-green axis:
Da* is positive or negative the trial is redder or greener than the standard.
Difference on yellow-blue axis:
Db* is positive or negative the trial is yellower or bluer than the standard.
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Tolerancing in L*a*b*
Color is measured
Limits are set
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Instrumentation
Spectrophotometer
Color fingerprint i.e. Reflectance spectrum